Standard NTSC (National Television System Committee) and PAL (Phase Alternation Line) televisions have a picture aspect ratio of 4:3. This means that the ratio of the width of the visible area to the height of the visible area is 4/3, or 1.33. HDTV (High Definition Television) specifies several formats that have a picture aspect ratio of 16:9, or 1.78.
If a program recorded with an aspect ratio of 16:9 is displayed on a device with an aspect ratio of 4:3 such that the entire width is visible, black bars are seen at the top and bottom of the picture. This is known as letterboxing.
If the original program material had 480 active lines (as in NTSC-Digital Video), then the area which contains the active image for a letterboxed program is only 360 lines, or 75% of the total viewable area. If this letterboxed program is recorded into a format that has an inherent resolution of 480 lines, then 25% of the recorded material is empty.
To improve this situation, a technique called anamorphic transfer was developed. In this process, the active area of the 16:9 program is stretched vertically to fill all 480 lines of the recorded material. To view this type of material, the display device needs to ‘squish’ the vertical dimension by 25%; exactly canceling out the stretch which was performed during recording. This has the benefit of putting 480 lines into a smaller vertical space, reducing the appearance of scan lines. If the source material is film, which has no inherent ‘line count’, the vertical stretch is accomplished optically, giving an increase in vertical resolution.
This process was originally developed and used on Laserdiscs and DVDs (Digital Video Discs), and many high end television sets have the capability of performing the 16:9 ‘squish’ to properly view this type of material.
The popularity of the ‘letterboxed look’ prompted several consumer DV (Digital Video) camcorder manufacturers to add a ‘16:9’ or ‘HDTV’ mode to their cameras. In order to do this correctly, the camera needs either an anamorphic element in the optical path in front of the CCD (Charged Couple Device) or photo sensitive sensor, or the elements in the CCD or sensor need to be shaped in an anamorphic fashion. This turns out to be very expensive to do, so most cameras ‘cheat’ to get this effect.
By taking the center 360 lines of the image and digitally stretching them to 480 lines, a result which is similar to having an anamorphic lens element is achieved. The drawback here is that all of the lines in the recorded material are actually manufactured lines, each one being a mix of two consecutive scan lines on the CCD. This does not give the increase in resolution that using an anamorphic lens adapter does.
In order to preserve the proper aspect ratio on the camera's viewfinder, the camera will perform a vertical ‘squish’ when displaying 16:9 material. This is detected in a DV camcorder by inspecting a bit in each frame in the DV stream. This ‘squish’ operation requires some amount of hardware to do on the NTSC or PAL output signal, so most consumer camcorders only modify the data displayed in their viewfinder. Some higher end decks, however, can also perform this ‘squish’ on the NTSC or PAL output, giving a letterboxed image on a television without the capability to do a 16:9 squish by themselves.
With a camera which has an anamorphic lens adapter attached, recording can be done as usual. The noticeable effect will be that in the viewfinder the image will appear stretched vertically, and the recorded image will only look correct on a monitor with a vertical ‘squish’ function.
For a DV camcorder set to 16:9 mode the material in the viewfinder will look correct, but the output of the camera will need to be displayed on a monitor with a 16:9 ‘squish’.
Editing can be performed on this material, as the anamorphic nature of the picture content does not affect access to the material. Some rendered effects will be fine, such as cross dissolves. However, any effects that generate shapes, or motion effects involving rotation will produce noticeable distortion in the image. Also, any composited graphics are likely to be distorted without careful aspect distortion. (All of these effects are distorted due to the change in the shape of each pixel that an anamorphic lens creates).